This image of Pluto’s largest moon Charon, taken by NASA’s New Horizons spacecraft 10 hours before its closest approach to Pluto on July 14, 2015 from a distance of 290,000 miles (470,000 kilometers), is a recently downlinked, much higher quality version of a Charon image released on July 15. Charon, which is 750 miles (1,200 kilometers) in diameter, displays a surprisingly complex geological history, including tectonic fracturing; relatively smooth, fractured plains in the lower right; several enigmatic mountains surrounded by sunken terrain features on the right side; and heavily cratered regions in the center and upper left portion of the disk. There are also complex reflectivity patterns on Charon’s surface, including bright and dark crater rays, and the conspicuous dark north polar region at the top of the image. The smallest visible features are 2.9 miles 4.6 kilometers) in size.

The first paper, from the University of Arizona, suggests that Sputnik Planitia filled with ice and altered tidal forces between Pluto and Charon.

The second, from the University of California, suggests the reorientation was caused by tidal forces as a result of a “slushy,” partially-frozen underground ocean.

New Horizons

Clues emerged from data gathered from NASA’s New Horizons space probe, which passed by Pluto in July 2015. Researchers from the Massachusetts Institute of Technology have been a part of the New Horizons team.

“A thick, heavy ocean, the new data suggest, may have served as a ‘gravitational anomaly,’ or weight, which would factor heavily in Pluto and Charon’s gravitational tug-of-war,” according to MIT News.

“Over millions of years, the planet would have spun around, aligning its subsurface ocean and the heart-shaped region above it, almost exactly opposite along the line connecting Pluto and Charon.”

Sputnik Planitia is thought to have formed billions of years ago when an asteroid slammed into Pluto’s surface, leaving behind a crater that filled up with nitrogen ice within the last 10 million years.

NASA’s New Horizons mission, which launched in 2006, captured images of this luminous region when it passed by the dwarf planet last year.